Wishy Washy Wuss MC

Seriously? He could have at least told her that he only found out recently and was debating whether or not to tell her. Would doing that make her forgive him? Probably not, but it would at least make sure she's not under the impression that he knew this whole time. Even if he couldn't talk to her in the moment, he could have passed on a letter through other people to communicate at least some of the details surrounding the incident as a way to show sincerity.

Like, yeah, his dad is the one who did it, but it was under the Emperor's orders. And there was at least a "for the greater good" reason for the orders. Knowing that probably wouldn't make her forgive the people involved; but it could reassure her that the direction of her revenge wasn't entirely wrong, since there was a powerful black mage involved who was planning on using the villagers as sacrifices. I'm fairly sure she's under the impression that there was no black mage involved and the incident was just another example of nobles not valuing the lives of commoners at this point.

If the black mage hadn't been in the village and hadn't been planning something shady, the village wouldn't have been wiped out. Conversely, if the empire had had the ability to deal with black mage in the first place, the incident also wouldn't have happened either. From a Utilitarian perspective and based on the circumstances at the time, it was the right call (assuming the black mage did actually die). She may not agree with the result of that decision, but at least give her the information so she can make an informed decision about who to blame. If she were told the circumstances surrounding the incident from the empire's perspective, she could at least take the full facts into account and not base her future decisions on misleading information and rushed conclusions.

She may also get some comfort from knowing that she's not the only survivor.